Recently, in Suitelets that I developed years ago with SuiteScript 1.0 have been giving problems. Specifically I noticed when I use response.setContentType('JSON'), I get Unexpected Error. Does anyone have any advice on this?

I don't recall how content types worked in 1.0, but I'd recommend either leaving that call out (letting it be inferred from your payload?) or setting it using 'application/json'